Transport of adequate numbers of normal spermatozoa through the cervical mucus is one of the crucial steps in the process of fertilization. It is well established that at midcycle this transport occurs very rapidly. It is also well known that the cervical mucus changes in both macrostructure and microstructure over the menstrual cycle, and at midcycle has a structure most conducive to sperm transport. However, the exact nature of the means by which the midcycle mucus assists in transport is still not clear. It may be a purely passive role, resulting from the mucus' channel-like structure at midcycle; however, there has been speculation that the macromolecular micelles of the midcycle mucus may actively assist the propulsion of sperm. To check the validity of this hypothesis high-speed cinephotomicrography of human sperm interacting with human cervical mucus will be used to analyze the tail motion of representative groups of sperm. Changes in the parameters characterizing the flagellar waves or in the velocity of travel will determine, qualitatively and quantitatively, the nature of the contribution of the cervical mucus.